Stress bro. Think of it this way. You are the snake and you are placed on the couch. you see a huge animal running buy trying to pick you up. It goes into defensive mode. Not mean....
try using a box or a tub to place him in while you work in the cage.
A bite from that size will break the skin but will scare oyou more then anything. It might be a one time thing too. My calmest snakes have their days.